1. Field of the Invention
The invention relates to the field of spacecraft and, in particular, to an improved ballute for decelerating the spacecraft such that it can be placed in orbit about an extraterrestrial planet or for actual reentry into the planet's atmosphere.
2. Description of Related Art
Deceleration of spacecraft for planetary orbit or re-entry into the planet's atmosphere creates severe temperature and pressure loads on the spacecraft. The space shuttle uses ceramic tiles, which provide excellent protection. It is especially suited for vehicles that are reusable, even though extensive maintenance procedures are required. Other single reentry spacecraft such as planetary probes have successfully employed heat shields made of ablative materials, while others have used metal heat shields. However, such systems impact the size and weight of a reentry vehicle. All are critical on spacecraft to be used for interplanetary exploration. Both directly impact size (and therefore cost) of the launch vehicle required to place a payload into orbit or to send the spacecraft to another planet.
Inflatable aerodynamic braking devices, commonly called ballutes, which have been in existence for a long time, offer a serious alternative. They have the advantage of being storable in a relatively small volume and, when inflated, can be expanded to a size many times in size. U.S. Pat. No. 4,504,031 "Aerodynamic Braking And Recovery For A Space Vehicle" and U.S. Pat. No. 4,518,137 "Aerodynamic Braking System For A Space Vehicle" both by D. G. Andrews disclose an inflatable braking device, which in the stored condition is mounted on the aft end of the spacecraft about a rocket nozzle. When deployed it produced a large aerodynamic braking surface. The rocket engine exhaust during braking provides a cooling layer of gas forward of the braking device such that it does not overheat. This will allow the spacecraft to enter a low earth orbit. However, it depends upon the use of a rocket engine to provide protective cooling gases. It would be unsuitable for a ballistic type reentry into the atmosphere required for landing the spacecraft on the planet's surface. This is primarily due to the high heat loads that would be introduced into the rocket engine as the spacecraft descended into the denser atmosphere. In fact, ballutes are generally designed for slowing a spacecraft into orbit about a planet, not for descent to the planet's surface. Heretofore the prior art ballute designs did not accommodate such a reentry. However, at least they offer a reduction in size and weight of the spacecraft when used in combination with a more conventional aerodynamic breaking device such as an ablative heat shield.
High temperature ceramic thermal blanks have proven useful on spacecraft. For example, fibrous silica bafting sandwiched between silica fabric and glass fabric. More recently, multi-layer materials have proven useful. For example, U.S. Pat. No. 5,038,693 "Composite Flexible Blanket" by D. A. Kourtides, et al. discloses a multi-layer insulation blanket. It comprises outer layers of Aluminoborosilicate (ABS), with Aluminum and Stainless Steel foils reflective layers there-be-hind. Spacers made of ABS or polyimide scrim were placed between the metal foils. An alternate approach used metal-coated polyimide cloth instead of metal foils. The various layers were sewn together with Silicon Carbide thread. Various combinations and numbers of layers were tested; all of which proved successful. If metal foils were used, a layer of ABS was inserted therebetween. If metal-coated polyimides were used they were aligned such that the metalized side contacted the polyimide side of the next layer. However, such insulation was not designed to be used in an inflatable structure. First of all, there is no specific gasbag layer. Inflatable aerodynamic braking devices or ballutes must be internally pressurized during reentry to maintain the aerodynamic shape. In addition, they must be flexible enough to store in a compact shape and expand to a comparably large aerodynamic shape. Also, they must be able to retain their physical shape and not grossly deform during entry/reentry, which would change its drag characteristics.
Thus, it is a primary object of the invention to provide an improved material for an aerodynamic braking device for a spacecraft.
It is another primary object of the invention to provide an improved material for an inflatable aerodynamic braking device for a spacecraft that allows the braking device to be stored in a small volume and which can be expanded to provide a large aerodynamic braking surface.